Portland cement, blast furnace slag cement, and other cement is broadly used for cement concrete for soil reinforcement, civil engineering structures, buildings, etc. The advantages of cement are that it is easy to make various shapes of structures by pouring into a mold either mortar or fresh concrete obtained by mixing a mixture of cement and an aggregate with water and that it is possible to produce concrete structures with high compressive strength. Further, it is possible to mix limestone or clay, which are available in huge quantities on the earth, with blast furnace slag, fly ash, or other industrial byproducts for use, so there is the advantage of the possibility of supply of large quantities at low cost. Due to these advantages, cement is one of the industrial products used in the greatest amounts.
In cement, blast furnace slag cement is a high vitrification rate granulated blast furnace slag (hereinafter simply referred to as “blast furnace slag”) which is finely ground (ground granulated blast furnace slag (GGBFS)) alone or a mixture of ground granulated blast furnace slag with Portland cement etc. Granulated blast furnace slag is a granulated material which contains a large amount of glass produced by water cooling blast furnace slag in a molten state at 1,300 to 1,500° C. Ground granulated blast furnace slag obtained by grinding this blast furnace slag by a grinding mill to a specific surface area of 3,000 (Blaine) or more, in high activity products, 4,000 to 6,000Blaine, is used as a cement material.
Note that, blast furnace slag is an inorganic substance of mainly ingredients produced as a byproduct when producing pig iron in an ironmaking blast furnace. In general, it contains SiO2 in 30 to 35 mass %, CaO in 40 to 45 mass %, MgO in 2 to 8 mass %, and, further, Al2O3 in 6 to 18 mass %. Further, as trace ingredients, it contains TiO2, CaS, FeO, etc. If the blast furnace slag has a vitrification rate of 95% or more, it is possible to produce good performance blast furnace slag cement.
If the water is alkaline, the CaO and Al2O3 contained in the ground granulated blast furnace slag will leach out from the granulated slag into the water causing a hydration reaction and contributing to solidification of the cement structure. However, under conditions where the water is neutral or acidic, the setting reaction of the ground granulated blast furnace slag will be extremely slow, so except for special cases, a mixed cement of Portland cement or another strongly alkaline cement with ground granulated blast furnace slag is used. In general, cement containing ground granulated blast furnace slag up to 30 mass % has substantially an equivalent function to the cement mixed with.
That is, the initial strength and the final strength of the solidified cement produced by this are substantially the same as the cement mixed with. This mixed cement can be used for applications in place of Portland cement in building and civil engineering fields. Further, cement containing ground granulated blast furnace slag in 30 to 70 mass % is slow in initial set of the solidified cement, but is high in final strength, is low in heat generation, and has other features. Due to this, it is used for large structures and civil engineering applications. In this way, it is possible to change the ratio of mixture of the ground granulated blast furnace slag in accordance with the application of the cement. Further, blast furnace slag cement is high in seawater resistance, has the effect of suppressing alkaline aggregate reactions, etc. Therefore, it is strong in durability even under adverse conditions and can be used for concrete for wavebreaker blocks, bridge trestles, etc.
Note that, blast furnace slag includes low alumina grades (Al2O3 content less than 10 mass %) and high alumina grades (Al2O3 content 10 mass % or more). High alumina ground granulated blast furnace slag releases many aluminum ions forming hydrates when the concrete solidifies. As a result, the concrete or mortar becomes higher in strength, so it is possible to produce good quality blast furnace slag cement from ground granulated blast furnace slag using high alumina blast furnace slag as a material.
In this way, blast furnace slag cement using high alumina ground granulated blast furnace slag has the feature of a high final strength of the solidified cement. However, in cement mainly comprised of high alumina ground granulated blast furnace slag and Portland cement, in soil containing sulfates, due to the effect of the sulfate ions, sometimes the concrete will expand over a long period of several years after solidification to 10 or so years after it. This is because aluminum ions are eluted from the alumina in the blast furnace slag and, further, calcium ions are eluted from the blast furnace slag and limestone contained in the Portland cement. These react with the sulfate ions to produce sulfates and, finally, form ettringite. This ettringite further reacts with the aluminum ions eluted and forms monosulfates of aluminum and calcium oxides. After the concrete solidifies, if sulfate ions further permeate the concrete, the monosulfates and sulfate ions will react and again form ettringite. At this time, the concrete will expand in volume, so the cement concrete will expand. In the worst case, the concrete will expand and cause the structure to be destroyed. In Japan, areas near volcanoes and some coastal areas have considerable sulfate soil. Further, overseas, dry belts such as the Middle East and the West Coast of North America have much sulfate soil. In these areas, the soil contains residual calcium sulfate, magnesium sulfate, sodium sulfate, etc. These sulfates corrode cement concrete resulting in easy occurrence of the problem of expansion and deterioration of the cement concrete.
To solve the problem of damage to concrete structures due to this sulfate expansion, low calcium aluminate Portland cement highly resistant to sulfate expansion is mixed with blast furnace slag cement for use.
Further, for applications where the effect of the sulfates is particularly large, sometimes the mixing ratio of the high alumina ground granulated blast furnace slag is made 60 mass % or more, preferably 70 mass % or more. That is, the ratio of Portland cement is lowered, the elution of calcium ions from the Portland cement is decreased, and the balance of the aluminum ions and calcium ions changes, so there are not enough calcium ions for formation of ettringite and therefore formation of ettringite is suppressed.
In mixed cement containing mainly high alumina ground granulated blast furnace slag and Portland cement, various measures are being taken to prevent sulfate expansion. For example, with mixed cement comprising low alumina Portland cement in which ground granulated blast furnace slag is mixed to 60 mass % or more, preferably 70 mass % or more, it is possible to suppress concrete expansion in a sulfate environment even more than the case of Portland cement alone, but the initial setting of the concrete was slow. As a result, it was only possible to use this for some civil engineering applications such as dams or embankments where the initial setting is allowed to be slow. Therefore, there was the problem that it was not possible to apply this for the production of concrete panels or tunnel segments or for building foundations.
In PLT 1, as the method for suppressing concrete expansion due to sulfates, sulfate ions for reacting with the aluminum ions initially eluted from the ground granulated blast furnace slag were introduced into the fresh concrete in advance. In this method, by forming the ettringite at an early timing, that is, before the expression of the concrete strength, it was possible to form ettringite after the concrete curing. Specifically, by adding a large amount of gypsum (CaSO4, anhydrous crystals and hydrated crystals in some cases) to blast furnace slag cement, expansion in a sulfate environment was suppressed.
However, in blast furnace slag cement containing ground granulated blast furnace slag to 10 to 60 mass %, even in the case of mixing in Portland cement with the greatest sulfate expansion inhibiting effect, it was necessary to add gypsum to the total cement weight in an amount of 4 mass % or more converted to SO3. However, the sulfate ions which are eluted from gypsum also have the effect of delaying cement setting, so if increasing the amount of addition of gypsum, there was the problem that the initial setting of concrete (within 1 to 3 days) was delayed. As a result, application to uses where fast setting is necessary such as building foundations or concrete panels or tunnel segments was difficult. Further, if adding a large amount of gypsum, there was also the problem of a drop in the final strength. To suppress this effect, it is necessary to make the amount of addition of gypsum 4 mass % or less converted to SO3. That is, in the prior art, there was no method for simultaneously achieving the conditions for the concrete setting speed of mixed cement made of high alumina ground granulated blast furnace slag and Portland cement and solving the problem of sulfate expansion.
PLT 2 and PLT 3 describe, as a method for production of concrete with high durabilities under conditions of a large presence of sulfate ions and further under an acidic environment, adding, in addition to the cement, 100 micron or less size granulated blast furnace slag, granulated steelmaking slag, and vitrification rate 10% or less slag aggregate. However, in this method, due to the formulation of materials at the time of installation of the concrete, the sulfuric acid resistance of the concrete was improved, but the sulfate resistant performance of the cement itself was not improved. Therefore, in this method, use was difficult in locations where only general aggregate can be obtained or for structures where it is necessary to use high strength aggregate.
By using blast furnace slag as a cement material, it is possible to make one of the byproducts produced by ferrous metal production, that is, blast furnace slag, into a high added value industrial material. This enables effective utilization of resources and energy conservation. However, to expand this application, it was necessary to raise the durability of blast furnace slag cement using high alumina ground granulated blast furnace slag in sulfate soil. Therefore, PLT 4 and PLT 5 were proposed for satisfying both this objective and the setting ability of cement.
PLT 4 proposes the addition of ground gypsum powder for the purposes of improving the setting function of blast furnace slag cement and lower the cost of increasing the fineness. Due to this, it was learned that by using gypsum powder with a high specific surface area, it is possible to create blast furnace slag cement having a performance equal to that of conventional blast furnace slag cement even if the ground granulated blast furnace slag is relatively small in specific surface area.
PLT 5 proposes control of the physical properties of a cement material by increasing the fineness of gypsum through the use of ground granules of high alumina blast furnace slag for the purpose of improving the durability in sulfate soil. It is important to increase the supply of sulfate ions at the start of setting, so increasing the fineness of the gypsum increases the dissolution speed and creates an excess supply of sulfate ions to thereby raise the initial settability.